current bias j-fet input 2sk170 ,2sj74 at input stage

My humble advice:

There are 3 types of 2sj74/2sk147: GR, BL and V. This tells you how big the Idss will approx. be. Look in the data sheet:

On page 2 you can see a curve called ID - VGS. From this I have concluded that one must have the signal as close to the top as possible to have minimal distorsion. And the smaller the signal is, the lower distorsion.

Unfortunately there are big differenties between the same types. One 2sj74 can measure Idss to 5 mA and the next 2sj74 can measure 10 mA. Therefore you must measure out all your fets and make sure they measure equal.

On you can read more about fets. In my opinion one should do this if one is not used to working with fets. Especially "The new Frontier" tells it all about bias and everything else.
In fact I also used them in the PR9R folded cascode preamp (released Jan 1995 Bangkok Hi Fi show) which used the FET complementary diff'l pairs, a bjt folded cascode and EF output, once again at about 2 mA each FET. It had a beautiful smooth open and detailed sound AND a (volume) remote and, I believe is still current in their range. Sold like hot cakes.


2004-12-25 12:24 pm
amplifierguru said:
Hi Kanwar,

I don't think so, You would best use the K170/J74 with your 2N5551/5401 cascode to take it up to such heights.

Thanks Greg,

But I have seen somewhere in one of the professional amps using 2 series connected Cascodes i.e. total 3 JFET pairs in series ..... does it suffice or only adds distortion or add something else......
Secondly how do you came to know about that we are using 2N5551/5401 pair in our designs...:)